A significant improvement in photopolymerization processing is realized when the chemical coating to be cured is blanketed by an inert atmosphere during exposure to ultraviolet radiation. The principle source of ultraviolet energy is a conventional mercury vapor lamp. Mercury vapor lamps are relatively inexpensive and relatively efficient as generators of electromagnetic radiation in the ultraviolet wavelength range.
In order to simultaneously provide a protective atmosphere at the surface of the coating while the surface is undergoing irradiation, it is necessary to house the mercury lamps in a confined enclosure in common with the inerting assembly. However, when one or more mercury vapor lamps, particularly lamps of high wattage, are located in the confined enclosure of the inerting assembly, sufficient heat is radiated to cause the ambient temperature of the enclosure to rise considerably. The elevated thermal environment will in turn precipitate failure of the lamps. Such failure has been attributed to deterioration of the conducting elements within the lamps and more specifically to oxidation of the molybdenum strips which are sealed to the quartz envelop at the opposite ends of the lamp and extend internally of the lamp to the tungsten electrodes. Since the inerting assembly is designed to pass inert gas into the confined enclosure, it would be only natural to assume that the design can be modified so that the inert gas provides the additional function of cooling the ends of the mercury vapor lamps. This would then be analogous to other known mercury lamp systems which pass air over and around the ends of the mercury lamps to provide cooling. Nevertheless to provide adequate cooling in this manner requires not only a relatively large flow of gas but an indefinite flow which will vary with the number of lamps used and their wattage characteristic and may not necessarily provide uniform cooling. Hence, although feasible, it is not compatible with efficient inerting assembly design. Furthermore the requirement of high gas flow is a serious economic disadvantage which might prove fatal to the commercial viability of a photocuring system dependent upon such flow. In fact, considerable research effort has been expended to design an inert gas blanketing system as taught in U.S. Pat. No. 3,807,052 entitled Apparatus for Irradiation of a Moving Product in which the required inert atmosphere is satisfied using a very low flow of inert gas and is one of the principle reasons why photopolymerization in an inert atmosphere has become commercially acceptable. The aforementioned patent discloses an inerting assembly including an enclosure having a treatment chamber, which houses the source of radiation, such as a plurality of mercury vapor lamps, and an inlet and exit tunnel extending from the treating chamber. The disclosure teaches the importance of the geometry and location of the inert gas injector and its orientation within the assembly to achieve dynamic inerting at low flows and emphasizes the importance of a non-turbulent flow of inert gas throughout the enclosure.
It was theorized that the problem of overheating at the sealed ends of the lamp could be prevented from within the lamp by coupling the electrically conductive elements at the opposite ends of the lamp to a heat exchange medium external to the lamp. Although this coupling can be carried out in a number of ways the most preferred is through a direct conductive coupling. This technique permits the direct transfer of heat from the molybdenum elements most susceptible to oxidation deterioration when the lamp is operating within an elevated thermal environment without affecting the operating characteristics of the lamp. It has been shown that by conductive coupling in a predetermined manner not only is seal failure prevented but the lamp is rendered substantially independent of the external thermal environment.